The national historic significance of the Sir John Johnson House lies in its historical association with Sir John Johnson, its age as one of the oldest surviving buildings in Ontario and in its architectural design.
In 1776, at the start of the American RevolutionaryWar, Sir John Johnson left his substantial estate in the Mohawk Valley in New York, and came to Canada. During the war, he organized, funded and led the King’s Royal Regiment of New York, which was involved in many border skirmishes. In 1784 he became responsible for the settlement of Loyalists who had fled the United States, in the area from Lake St. Francis to the Bay of Quinte. He was given a substantial grant of land for himself, part of which included the land on which the hamlet of Williamstown is located today. On his Williamstown property, sometime between 1784 and 1792, Johnson had built a one and a half story, five-bay log structure on a rise of ground, near the Raisin River. A grist mill and a saw mill were built on the river in the same period.
While there is little to suggest Johnson ever lived in his Williamstown house, the mills were the focal point in his vision for the community he hoped to encourage at that location. The house was occupied by the overseer of the mill for at least part of Johnson’s ownership of the site. The features which can be associated with this period include the original portion of the house and the sites of the former mills.